Stators and rotors for electrical rotating machines are commonly produced by softening bulk metal and then shaping it. Softening may be done with heating while shaping may be done with a number of methods, including stamping, laminating, and annealing. These methods enable the motor to be precisely shaped. Electrical rotating machines may be mass-produced, since the methods are easily repeatable.
In conventional spindle motors, stators and rotors have been made by lamination. In this method, thin sheets of metal—called lamination stock—are cut into shapes by stamping, laser cutting, or equivalent techniques. The lamination stock may include a thin insulating material, possibly augmented after stamping. The stamped pieces of sheet metal are called laminations, and these are stacked to form the rotor or the stator.
When a stator is formed, the laminations are generally circular. If the completed stator is to have poles extending from its inner or outer surface, the circular laminations include protrusions which form the poles when stacked. Wire is then wound around the poles to form stator windings. When a rotor is formed, holes are left in the lamination stack for the various components, such as wires and rotor bars. The central shaft is pressed into the lamination stack, and the rotor bars pressed or cast into the edges.
The conventional method of forming motor components has a number of drawbacks. First, most steel is manufactured in rolled sheets and thus has a grain orientation. The grain orientation has an effect on the magnetic flux properties of the steel. In a circular stamped piece of steel cut from a sheet with a grain orientation, the grain orientation will be aligned with one diameter of the circle, transverse to the perpendicular diameter of the circle, and at varying angles with the other diameters of the circle. The inconsistent alignment of the grain structure around the circle causes magnetic flux values to differ in parts of the component and thus the motor does not have consistent and uniform torque properties as it rotates. The problem of grain alignment consistency may be eliminated, at a cost, by appropriate annealing and forming techniques, or by stacking the laminations in such a way that grain orientation is evenly distributed around the stator or rotor.
Another drawback with using circular steel pieces is that, especially for inward facing poles, it is difficult to wind the wire windings tightly because of the cramped space inside of the laminated stator body. The need for adequate working space creates a lower limit on the size of the stator and thus the motor. The limited working space also results in a low packing density of wire. The packing density of wire coiled around the poles affects the amount of power generated by the motor. Increasing packing density increases the power and thus the efficiency of the spindle motor.
An important factor in motor design is to reduce stack-up tolerances in the motor. Stack-up tolerances reduce the overall dimensional consistency between the components. Stack-up tolerances refer to the sum of the variation of all the tolerances of all the parts, as well as the overall tolerance that relates to the alignment of the parts relative to one another. One source of stack up tolerances is from the circular stator body. Generally, the thickness of rolled sheet steel is not uniform across the width of the sheet. Sometimes the edges are thicker or thinner than the center. In a stator made from circular stamped pieces, the thicknesses of individual laminations are thus different from one side to the other. When stacked together, this creates a stack up tolerance problem. Furthermore, the circular stampings leave a lot of wasted steel that is removed and must be recycled or discarded.
U.S. Pat. No. 5,672,927 to Viskochil discloses a spindle motor having an overmolded stator. One drawback with the overmold used in this patent is that it has a different coefficient of linear thermal expansion (“CLTE”) than the corresponding metal parts to which it is attached. Another drawback with the overmold is that it is not very effective at dissipating heat. Further, the overmolds shown in this patent are not effective in damping some vibrations generated by energizing the stator windings.
U.S. Pat. No. 5,806,169 to Trago discloses a method of fabricating an injection molded motor assembly. However, the motor disclosed in Trago is a step motor, not a high torque induction motor. Furthermore, neither Viskochil nor Trago addresses the problem of variations in the thickness of steel used to make stator cores and the non-uniform grain structure in the steel causing variable magnetic flux in the stator during operation of the motor.
U.S. Pat. No. 6,892,439 to Neal et al discloses a motor including a stator having multiple conductors that create a plurality of magnetic fields when electrical current is conducted through the conductors. The stator has a pair of opposing end surfaces in contact with each other forming a toroidal core. A monolithic body of phase change material substantially encapsulates the conductors and holds the toroidal core in place. The stator is formed by laminating strips together to form a linear core preform, winding wire around poles extending from a side of the core preform, then rolling the preform to bring its two ends together to form the toroidal core.
In order to produce a motor permitting extremely high torque, it is desirable to create the magnetic sections of the motor with a material that improves upon the magnetic properties of pure iron. One solution is to melt two materials and subsequently combine them. This adds significantly to the time and cost of manufacture and the components may not blend satisfactorily. Another solution is to place two materials adjacent to one another. However, since the materials are not combined, the properties of each material are not present in the position of the other material. An alloy may be used, but alloys with high magnetic permeability are commonly too soft to be built into a strong motor as laminations.
A need exists for an improved high torque motor, overcoming the aforementioned problems of grain structure inconsistency, stack-up tolerances, and waste of material. In addition, there is a need for a method of producing a motor that works both with conventional silicon steel and with softer alloys having higher magnetic permeability.
The hot isostatic pressing (HIP) process, which subjects a component to elevated temperatures and pressures to eliminate internal microshrinkage, has enabled engineers to design components so they could meet specifications for use in critical, highly stressed applications.
The HIP process provides a method for producing components from diverse powdered materials, including metals and ceramics. During the manufacturing process, a powder consisting of one or more components is placed in a container, typically a steel can. The container is subjected to elevated temperature and a very high vacuum to remove air and moisture from the powder. The powder in the container is then subjected to high temperature and high pressure, the latter achieved by injecting high-pressure inert gas through an opening in the container. This process results in the removal of internal voids and creates a strong metallurgical bond throughout the material. The result is a clean homogeneous material with a uniformly fine grain size and a near 100% density. Typical approaches are disclosed in U.S. Pat. No. 6,210,633 and U.S. Patent Appl. Pub. No. 2004/0081572.
Motor components made of powdered material are known in the art. U.S. Pat. No. 6,956,307 to Engquist, et al. discloses motor stators formed in components by powder metallurgy and then assembled. U.S. Pat. No. 6,300,702 to Jack, et al. discloses a stator made of magnetic powder material and U.S. Pat. No. 6,651,309 to Gay et al. discloses a stator core made from magnetic powder material for use in alternating current generators and electric motors. U.S. Pat. No. 6,952,065 to Park, et al. discloses a laminated stator with poles formed by powder metallurgy. U.S. Pat. Nos. 6,811,887 and 6,432,554 to Barber, et al. discloses a system and method where powdered material is placed within a container along with an insulated coil which, when energized, subjects the powdered material to dynamic magnetic compaction and forms it into an electrical component. U.S. Pat. Nos. 5,982,073, 6,129,790, 6,251,514, 6,309,748, 6,340,397 and 6,342,108 to Lashmore, et al. disclose composition of a ferromagnetic powder for forming soft magnetic parts. U.S. Pat. No. 6,117,205 to Krause et al. discloses corrosion-resistant, soft magnetic metal components manufactured by powder metallurgy and infiltration processes.
Advantages of materials produced by HIP include reduced porosity, which enables materials with improved mechanical properties and increased workability to be produced. The HIP process eliminates internal voids and creates clean, firm bonds and fine, uniform microstructures. These characteristics are not possible with welding or casting. The virtual elimination of internal voids enhances the performance of the part produced by this technique and improves fatigue strength. The process also results in significantly improved non-destructive examination ratings.
A further advantage of the HIP process is its ability to create near-net shapes that require little machining. Conventional manufacturing methods use only 10-30% of the material purchased in the final product; the rest is removed during machining. A HIP'ed near-net shape part typically uses 80-90% of the purchased material. As a result, machining time and costs are significantly reduced. The strong combination of improved raw material use and greater machining efficiency that results from the HIP process has driven the growth of HIP'ed powder metal parts manufactured from nickel-based and titanium-alloys.
HIP has been applied to a number of industries, including automotive (turbocharger wheels and diesel engine valve lifters), medical (prosthetic devices), petroleum (valve bodies) and chemical processing.